1. Field of the Invention
The present invention relates to the field of material aging research and management, specifically to the in-situ monitoring and estimation of the condition of various degradable components used in a wide variety of applications (including, inter alia, electrical cable, process system valves, aircraft, spacecraft, and automobiles) via neutron activation techniques.
2. Description of Related Technology
The aging of degradable components (particularly those constructed in whole or in part of organic compounds such as polymers) is of great importance to modem society. Such degradable components comprise a significant fraction of what may be termed as "critical" components in use in many industrial, aerospace, and automotive applications, both commercial and military. Included in this category are components such as electrical cable insulation, valve internals, bushings, seals, and gaskets. Degradation and ultimate failure of these so-called critical components is of paramount importance in that such failures may result in the unanticipated maintenance costs, loss of operational capability and availability, and even loss of human life.
Several different approaches to managing the aging of such components exist. One approach involves 1) subjecting laboratory or in-situ specimens of a given component to a progressive regimen of aging stressors such as heat, radiation, electrical potential, chemicals, and/or oxygen present in the anticipated operating environment (known generally as "artificial aging"); 2) identifying a critical parameter of the component's function in the desired application (such as dielectric strength for an insulator); 3) determining a maximum or minimum acceptable value for the chosen parameter; 4) correlating the maximum or minimum acceptable value to a given installed lifetime (for example, via aging models such as the Arrhenius equation); and 5) removing the component from service when the installed lifetime is reached. Note, however, that this approach has the distinct disadvantage of not directly monitoring the condition of a given component, thereby introducing potentially significant variations in component condition across various applications. Specifically, some applications may have aged more or less than expected (due to a variety of factors such as radiant heat or radiation shielding, variations in oxygen/inert gas concentration, aging prior to installation, inaccuracies in the aging model used, etc.), and hence are being replaced either prematurely or too late. More effective condition monitoring programs will utilize a similar approach as that outlined above, yet instead of rotely replacing a component at a given point in life, will monitor the degradation of the component as a function of time to determine it's rate of aging as compared to the artificially (or naturally) aged specimen. The primary drawbacks of these latter condition monitoring programs include the costs of monitoring, component inaccessibility, and component/device downtime. For example, the condition monitoring of a fluoropolymer valve seat requires either remote inspection or disassembly of the valve, thereby removing the valve from service for a period of time. In such cases, simple periodic replacement of the component during other scheduled maintenance may be more cost effective. In some instances (such as electrical cable, described further below), no periodic maintenance or replacement is ever scheduled; hence condition monitoring of some sort is almost a necessity. The enormity of cost associated with replacement of cable in, for example, a commercial nuclear power facility, underscores the need for effective aging assessment and monitoring techniques.